too much negative pressure

I have a 8in inline fan hooked to my lights inside the tent blowing air out of the tent. The fan only runs 471 cfm. I also have two 8 in ducts coming in from my HEPA filter from outside. The filter is rated for 1300 cfm and is 6in. I have so much negative pressure the tent is getting sucked in about 3-4 in per side. If I open 1 half moon window on the bottom I have no problem with the pressure. Please help I need the space in the tent I'm losing. What am I doing wrong?

I've always been told to have double the size of intake for exhaust. so 8" would need 16" or two 8" for intake to equal out.

put a controller on ya big fan, turn it down.

Get a fan speed controller and turn your fan down.

I have this same problem with my small tent. So I bought a speed controller but when i turn the fan down so the sides of the tent aren't sucking in, the temps start to rise. If i turn the speed up, temps are ok, but the sides of the tent suck in. I'm using a 6" inline exhaust and a 6" passive intake. I think if I get a fan hooked up to the intake I might be able to find a happy middle ground

then make the passive intake bigger ..

I have two, one that "collect" fresh air from another room with a little help from some dutching..

and one I can open and close using a DIY cardboard brick, of course light proven with some more dutching running inside the grow room, and to the place I want it, right by my plants ..

works for me, I just close tube No 2 just enough to see a bit negative pressure, so I know all air going out is through my filter, and no smell escapes

1300 cfm from a six-inch fan? Wow. Now I'm gonna spend the rest of the day looking for a bizjet flying in circles ... cn

Too much negative pressure? Simple solution, bigger intake hole.

My mind is bouncing all over the place with that sweet straight line! cn

I have a similar problem, but I made an entire bedroom a ”grow room”, do you keep the 8” hood fan on at all times, or do you turn it off during lights out?

Apparently I didn't write correctly or clearly My Fan is 8 in inline fan pulling 371 cfm not 1300. my HEPA filter is 6 in rated for 1300 cfm. All my duct in is 8 in. 1 exhaust and 2 intake all 8in. Ducting.there is a wye at the hepa filter which is also 6 in(6 in wye with two 8in reducers) if I knew how to post pics ffom my phone I would . The fan is also only on when the light is on. I also have a speed controller already and I run into the heat problem if turned to low.

Wait do you have the hepa filters hooked up to the intake or the exhaust, if they are hooked up to the intake that is probably your problem hepa filters are very dense because of how well they filter so if you have them on your intakes they are probably what is causing you negative air pressure problem because your intake can not keep up with your exhaust.

steiny88 said:

I have a 8in inline fan hooked to my lights inside the tent blowing air out of the tent. The fan only runs 471 cfm. I also have two 8 in ducts coming in from my HEPA filter from outside. The filter is rated for 1300 cfm and is 6in.

Click to expand...

Why are you HEPA filtering the air? I feel ya' when the desire is to supply clean air to the tent, but a HEPA is a bit of an overkill and may just be the reason for the issue. The CFM rating on that filter only indicates that the filter will perform it's function when UP TO 1300 cfm is moved across the thing. The filter manufacturer is telling you that moving more than 1300 cfm of air across the filter and the filter (or your ventilation system or both) won't perform properly.

What's most important to you is the pressure drop across the filter, i.e. how much suction you have to apply to get the air across the filter. Clearly, the HEPA is restricting the makeup air intake through your intake ducts. Even though your fan can MOVE over 400 CFMs does NOT mean that the fan will develop the suction/static pressure to overcome the filter restrictions.

Growers often base ventilation decisions on CFMs alone, and neglect suction/static pressure and its affect on the system from what I've been reading on these boards.

Lose the HEPA. You're not running a clean room, just a grow room.

I've taken the HEPA filter off and it makes no difference. I am also trying to keep my grow room clean also so I would like to use it if possible

How much ducting do you have on your intakes? A couple of feet? More?

Please find a way to post pics; you'll get the most help w/ pics.

371 cfm is too much for a small room to run all day, even for 15 minutes IMO. A fan that size would be good for a room thats at least 10x10. (I am basing this off of HVAC though.) and your intake should be the same as your exhaust (cfm wise not size). Only way for the negative pressure to stop is if the intake is bigger or equal than the exhaust(also cfm wise not size). I would go 10" minimum on the intake though. Just because the fan has an 8" opening doesnt mean that an 8" hole would work. 8" flex will only allow 200 cfm based on a .08 friction loss. 10" will allow for a max of 400 cfm. As for the hepa filter, I agree about the filter will only work up to 1300 cfm. If the hepa is not pulling at least 380 cfm into the tent then you will continue to get negative pressure.

fso heres pictures sorry i couldnt get them up earlier i could not upload them from my phone.

Your gonna have to seperate the 2 lines. When the air is being vented out from the fan you restrict fresh air from comin in through the same filter. That filter your using can only be one or the other. Another thing is the Y is only gonna let you push/pull so much air through it, seeing as how you upsized the duct and the Y is smaller. Air is almost like electricity, it goes through the path of least resistance. WHenever Using a Y to push the air out, you generally want to use the same size as the duct or bigger. If you have 2 10" flex you dont want to go down to a 6" Y. When you start undersizing fittigs you start to restrict airflow and/or you start to hear a loud noise due to the air. I say lose the Y and see if the negative pressure stops but make sure that your fresh air duct is bigger than your exhuast duct.